Systemd/Packagekit Integration

With GNOME moving to using more and more systemd and PackageKit in
different places, instead of having to patch all the apps using them to
use our services, it would be great to implement those in our stack
(aptdaemon and ubuntu-system-service).

Work has been started to implement PK system bus interface in aptdaemon,
so part of this is in progress already. Also, systemd interfaces needed
so far are just a couple easy ones AFAIR, so shouldn't be hard at all.